It's because when you switch back to normal you have to cycle through the different modes. You will have to go to through thermal which is then recognised as disabled and therefor removes it off your gun to prevent exploiting. At least that is what I have found.

I myself am a Lieutenant and would just like to question this. Were you guys harassing the cop and showing no purpose just to be annoying? Were you previsouly warned? And when did this occur?
Obviously you could lie about not harassing but just be fair and respond truthfully please.
Cheers
-Lt. Reaver